1. Field of the Invention
The present invention relates to adhesive compositions, and more particularly to alpha-cyanoacrylate adhesive compositions.
2. Description of the Prior Art
Alpha-cyanoacrylates are in wide use as instantaneous adhesives but have a low viscosity of about 2 about 3 cps, so that it is often desired to thicken these compounds to a desired viscosity for the convenience of use.
For example, it is attempted to dissolve an organic polymer, such as polymethyl methacrylate or hydroxyethyl cellulose acetate, in an alpha-cyanoacrylate to impart viscosity to the compound.
However, the composition incorporating such an organic polymer is a fluid having a cobwebbing property. For more convenient use, therefore, techniques have also been developed for preparing a thixotropic pasty composition comprising an alpha-cyanoacrylate and a hydrophobic fumed silica.
Already known as hydrophobic fumed silicas are those which are prepared by treating the surface of a hydrophilic fumed silica with polydimethylsiloxane, trialkoxyalkylsilane, hexamethyldisilazane or dimethyldichlorosilane.
When the hydrophobic fumed silica obtained by the treatment with polydimethylsiloxane or hexamethyldisilazane is admixed with alpha-cyanoacrylate in an amount of 5 to 8 wt. % based n the resulting composition, the composition obtained is thixotropic, pasty and about 30000 to about 70000 cps in viscosity, whereas the hydrophobic fumed silica as treated with dimethyldichlorosilane affords only a flowable composition having a viscosity of about 200 to about 900 cps even if incorporated therein in an amount of 8 to 15 wt. %.
In fact, the specification of U.S. Pat. No. 4,533,422 states that use of such a hydrophobic fumed silica obtained by the treatment with dimethyldichlorosilane merely provides a composition of low viscosity. Thus, the combination of alpha-cyanoacrylate and hydrophobic fumed silica as treated with dimethyldichlorosilane fails to give a nonflowable pasty composition.